Parts per Million, Parts per Billion

The constituents that are dissolved or suspended in water, whether they are natural minerals or serious chemical contaminants, are usually measured in either “parts per million” or “parts per billion.” Another way of expressing the same values is “milligrams per liter” or “micrograms per liter.”

There are a variety of ways to express these same values.

mg/L or ug/L

ppm or ppb

are the same.

The u is sometimes written as the Greek letter µ, but the meaning is the same.

When you read a water test, the first thing you need to notice is what the unit of measurement is. Is the contaminant being reported as parts per million or parts per billion? To say the least, this is important, because one part per million represents a quantity one thousand times greater than one part per billion. I won’t bother with the old “if you cut a pie into a million pieces” explanation.

To convert parts per billion to parts per million, divide by 1,000. If the EPA’s maximum allowable (MCL) for an industrial chemical is 2 ppb, dividing 2 by 1,000 gives you the allowable in ppm: 0.002.

If a water test reports in mg/L, you get the ug/L by multiplying by 1,000. If a test finds 0.015 parts per million of a substance, it’s the equivalent of 15 parts per billion.

Parts per trillion, by the way, is usually expressed as ng/L (nanograms per liter) and 1 ug/L = 1000 ng/L.

It is hard to think through the relationships between water contaminants because they vary so greatly and are seldom expressed in the same denomination. For example:

The current “allowable” (MCL) for nitrates (which many cities in Iowa are finding hard to meet) is 10 parts per million. That’s 10,000 parts per billion, or 10,000,000 parts per trillion.

The current allowable for arsenic is 0.010 parts per million. That’s 10 parts per billion or 10,000 parts per trillion.

The current allowable for lead is 0.015 parts per million. That’s 15 parts per billion or 15,000 parts per trillion.

There is no national allowable for the likely carcinogen 1,2,3 -Triclopropane (TCP), but California’s proposed limit is 5 parts per trillion. That’s 0.005 parts per billion or 0.000005 parts per million.